The primary focus of this work has been the mechanism of protein biosynthesis in the mouse plasmacytoma. Studies have involved: (1) incorporation by plasmacytoma microsomes of methionine from fmet-tRNAf; (2) analysis of the products of this incorporation by acrylamide gel electrophoresis; (3) studies on the regulation of plasmacytoma protein biosynthesis by transfer RNA; (4) studies on the synthesis of L-pyroglutamic acid. It has been found that methionine from fmet-tRNAf can be incorporated by plasmacytoma microsomes into a product with the electrophoretic mobility of immunoglobulin light chain. No evidence for a higher molecular weight precursor of light chain has been obtained. Chromatographic analysis of the seryl-and threonyl-tRNA's from two plasma cell tumors has revealed extensive differences in the nature of the isoaccepting species present in the two. Some evidence for the regulation of the rate of protein synthesis by plasmacytoma tRNA has been obtained. The synthesis of L-pyroglutamic acid from glutamic acid has been demonstrated in a cell-free protein synthesizing system from the plasmacytoma.